How to Calculate Percent Composition
Step-by-Step Method
- Find the molar mass of the entire compound by summing the atomic masses of all atoms in the formula.
- Determine the total mass of each element present in the compound using the atomic mass and the number of atoms of that element.
- Divide the total mass of each element by the molar mass of the compound.
- Multiply the result by 100 to convert it into a percentage.
Formula used:
Percent Composition = (Mass of Element / Molar Mass of Compound) × 100%
Worked Example: Percent Composition of H2O
Step 1: Molar Mass of H2O = 2(1.01) + 16.00 = 18.02 g/mol
Step 2: Mass of H = 2 × 1.01 = 2.02 g
Step 3: Mass of O = 16.00 g
Step 4:
Percent H = (2.02 / 18.02) × 100 ≈ 11.2%
Percent O = (16.00 / 18.02) × 100 ≈ 88.8%
Helpful Tips
- Always use atomic masses with at least two decimal places for better accuracy.
- Double-check the number of atoms of each element in the formula.
- Percentages should add up to approximately 100% (slight rounding differences may occur).